If no external force acts on the system

a) Acceleration of centre of mass is zero

b) Velocity of centre of mass is zero

c) Centre of mass may be at rest (or) moving with constant  velocity

d) Velocity and momentum of centre of mass remains constant

Without a force, there is no acceleration, which implies steady speed. 

Kinetic energy also stays constant because velocity is constant. 

The change in moment of inertia may cause the angular momentum to alter. 

Additionally, due to a change in potential energy, the total mechanical energy may also vary.

The total momentum of a system with constant mass that is considered isolated maintains a constant value over time.

Hence the correct answer is a, c and d are true.